The PPC970 allows events from the ISU to be selected in two differentways. Rather than use alternative event codes to express this, weinstead use a single encoding for ISU events and express theresulting constraint (that you can't select events from all threeof FPU/IFU/VPU, ISU and IDU/STS at the same time, since they all comein through only 2 multiplexers) using a NAND constraint field, andwork out which multiplexer is used for ISU events at compute_mmcrtime.